Useful tips for conserving leaf litter are also included. Home landscapes, especially woodland gardens, will benefit too. Confused about the terminology associated with native gardening? Hopefully the following explanations will clear up some confusion!
Then you search another nursery, another garden center, and another, and another — in an endless, futile search for a plant that is supposed to be…. A non-native species, honey bees were first brought to North America in by….
However, our findings indicate that we need to integrate litter-feedback effects into the framework of plant-soil feedbacks. Figure 2. Shoot dry weight A and root dry weight B of Betula pendula and Festuca rubra plants in response to soil source the plant type underneath which the soil was collected in the field and litter incubation treatments the litter type that was incubated in the soil during the experiment. C Schematic overview of the experimental set up.
Soil samples used in this experiment were collected in the field underneath Betula and Festuca plants left column. Then soil samples were incubated under controlled conditions with plant litter from both plant species using a full-factorial reciprocal transplant design middle column. Finally, we grew seedlings of both plant species on each soil in a full-factorial design right column. Here, we propose a novel framework that integrates rhizosphere- and litter-mediated PSF effects Figure 1. In this paper we will focus on the role of litter-mediated PSF effects as there are already detailed reviews and meta-analyses focussing on rhizosphere-mediated PSFs e.
We explore three main pathways via which plant litter can drive PSFs: physical, chemical and biological Ehrenfeld et al. For each of these pathways, we describe how they may contribute to explain PSF effects. It is important to note that even though we separate the litter-mediated PSFs via the three different pathways, under natural conditions, effects are often hard to disentangle as PSF effects will be mediated by interactions between the pathways.
In addition, we identify ways forward in PSF research to increasing our understanding of interactions between litter-mediated and rhizosphere-mediated PSFs, which will advance our knowledge of PSFs in natural ecosystems.
Physical pathways have received little attention in research on litter-mediated PSFs, but it has long been known that plant litter has strong effects on the physical soil environment Facelli and Pickett, a , both in natural Facelli and Pickett, b and agricultural ecosystems, i. The effects of plant litter on the physical soil environment have strong potential to feed back to plant performance through effects on seed germination, seedling establishment, and initial plant growth Olson and Wallander, ; Asplund et al.
A layer of leaf litter may improve the microclimatic conditions for seed germination through moisture retention and buffering against temperature extremes. However, under field conditions the evidence for improved germination rates in the presence of a litter layer is mixed e. Further, soil temperature is generally higher under a layer of plant litter Sharratt, , because the build-up of litter on the ground surface affects the transfer of heat between the soil and the atmosphere, which in turn can lead to increased seed germination rates Paul et al.
In contrast, the litter layer can also have negative impacts on seed germination and plant growth, because it can reduce the amount of light reaching the soil surface Facelli and Pickett, a and it may act as a physical barrier to seedling emergence Barrett, Finally, incorporation of litter into the soil matrix may modify soil structure, which may modify plant growth responses. Although most evidence for physical effects of litter on plant performance comes from studies on leaf litter, there may also be important effects mediated via root litter Bardgett et al.
As root litter is already incorporated into the soil matrix, it may not form a litter layer and therefore may have different physical PSF effects than leaf litter. Litter-mediated PSFs via the physical pathway have not been studied extensively, but may be plant species-specific. The retention of soil moisture underneath leaf litter layers is strongly related to the traits of the plant litter species, for example specific leaf area SLA , three-dimensionality of the litter, or tensile strength Swift et al.
Also, plant litter may have species-specific effects on soil water repellency, depending on the presence of organic hydrophobic compounds Cesarano et al. Similarly, light extinction curves in litter layers differ between litter species Facelli and Pickett, b.
The empirical support is scarce, but these litter-specific effects on environmental conditions strongly suggest that litter-mediated PSFs via the physical pathway have species-specific effects on seed germination and seedling growth.
Plant responses to litter-induced changes in environmental conditions via the physical pathway may also be species-specific e. In many empirical experiments it remains hard to untangle physical litter PSF effects, from chemical or biological effects.
However, measurements on e. For example, using a controlled pot experiment testing effects of litter on seedling emergence, Donath and Eckstein found significant interactions between litter type and plant species origin at least partly driven through mechanical effects.
Woodland species produced more biomass in presence of oak litter than in presence of grass litter, indicating a positive litter-mediated PSF effect which was explained by oak litter consisting of individual leaves, while grass litter typically forms dense interwoven mats which might be difficult to penetrate and could inhibit shoot emergence. For mechanical inhibition, seed size and seed position can be important in determining the strength of physical litter feedbacks Donath and Eckstein, Zhang et al.
Also, litter accumulation by an exotic plant species Avena fatua reduced the germination of small-sized seeds of native species via increased litter depth and light reduction, thereby facilitating invasion of A. The impact of light reduction suggests that at least physical pathways may play role, but to what extent other pathways also play a role remains to be tested.
Together, these plant-specific impacts on and responses to the litter layer show that the creation of physical barriers and alteration of the abiotic environment by plant litter may play a key role in PSF in natural ecosystems.
Understanding how shoot and root litter traits drive such physical litter-mediated PSFs may offer a promising avenue for further exploring the role of litter in PSF Bardgett et al. However, only few empirical studies have specifically focused on the role of physical effects in driving PSF. This is experimentally challenging, but may provide insights in plant growth responses to litter inputs.
It is important to mention that the level of specificity of physical litter-mediated PSF may be lower than for rhizosphere-mediated PSF, because the strength and direction of physical litter feedbacks may be largely determined by generic leaf and seeds traits; however, this warrants further investigation. During decomposition a wide range of chemical compounds is released from plant litter which can be beneficial or detrimental to subsequent generations of plants Facelli and Pickett, a.
Litter-mediated PSFs via the chemical pathway can be driven by the liberation of plant nutrients, secondary metabolites or DNA from decomposing litter Facelli and Pickett, a ; Mazzoleni et al.
Soil nutrients can be made available through rapid decomposition of nutrient-rich litter. This will increase plant nutrient availability in the next generation.
In contrast, litter with a high concentration of structural carbohydrates, such as cellulose, decomposes slower and may produce less positive or even negative PSFs Vahdat et al. Although litter-mediated PSF via nutrients may be less species-specific than PSFs mediated via other chemicals, plant species responses to litter-mediated changes in soil nutrient cycling often differ between plant growth strategies.
Generally, fast-growing, exploitative plant species will benefit most from positive litter-mediated PSFs via nutrient availability, while plant species with conservative resource-use strategies may be less hampered by slow recycling of nutrients Wardle et al.
Also, positive litter-mediated PSFs may favor the invasion of exotic plants, that are often better competitors for nutrients released from plant litter Eppinga et al. However, generalizing litter-mediated PSFs through nutrients may be complicated because of confounding effects of other chemical compounds or biotic interactions, which may result in unexpected effects on future generations of plants Hobbie, Plant litter also contains a range of secondary metabolites, including alkaloids, phenolic compounds and terpenes, which are often used as a defense mechanism against herbivores and pathogens and therefore play a key role in plant-microbe and plant-herbivore interactions Chomel et al.
These secondary metabolites are important for determining decomposition rates. Yet, it is now increasingly acknowledged that some complex compounds, such as lignin, can be degraded more quickly than previously assumed Lehmann and Kleber, , but that this may depend on the presence of specialized microbial communities, such as certain lignin-degrading fungi van der Wal et al.
As a result, secondary metabolites can contribute to litter-mediated PSFs via impeding nutrient cycling Wardle et al. Secondary metabolites can also have direct impacts on plant growth when released into the soil, and their impact may strongly depend on the residence time in the soil Chomel et al.
Many chemical compounds that are present in the living plant, such as alkaloids, are quickly metabolized in litter and are thought to have limited effects on soil processes and plant responses Siegrist et al. However, other chemicals liberated from plant litter can persist in the soil after senescence and may inhibit the growth or germination of neighboring and next-generation plants Bonanomi et al.
Also, flavonoids, which are known to play a role in attracting beneficial microbes, such as Rhizobia , may remain in plant tissue after senescence and affect plant growth by scavenging free radicals and improving stress tolerance Barazani and Friedman, In addition to secondary metabolites, DNA released from decomposing material may also hamper plant growth of next generations of plants, and this negative feedback is specifically targeted toward congeneric plant species Mazzoleni et al.
Litter-mediated PSFs via chemical compounds may strongly differ between above- and belowground plant organs. Although decomposition rates of shoots, stems and roots broadly correlate across large-scale fertility gradients, at the level of sites or individual plant species decomposition rates may differ between plant organs Hobbie et al.
This means that above- and belowground plant organs, as well as different types of roots, may play a different role in ecosystem processes Freschet et al. For example, leaf litter decomposes often more rapidly than root litter Freschet et al. Yet, root decomposition is likely to play a key role in subsurface soil layers and in ecosystems, such as tundra and grasslands, where half to three quarters of plant biomass is produced belowground as roots Poorter et al. Therefore, it is essential to start disentangling the role of root and shoot litter driving chemical litter-mediated PSF effects.
Local variation in decomposer community composition and activity drives variation in decomposition rates Hattenschwiler and Gasser, ; Vos et al. Variation in decomposer composition may be tightly linked to plant composition Bezemer et al. The litter-fragmenting community or detritivores , including earthworms, millipedes, myriapods, diplopods, and various insect larvae, transforms a large part of plant litter into feces, thereby increasing the surface area for microbial decomposition, which often results in accelerated litter breakdown Hattenschwiler and Gasser, ; David, ; Joly et al.
The impact of litter shredders on decomposition may be strongly affected by decomposers actively selecting certain litter types or compounds. For example, earthworms often prefer palatable residues, such as litter with low carbon:nitrogen ratios and tend to avoid ingestion of root litter Curry and Schmidt, ; Vidal et al. In addition, the functioning of the gut microbiome of litter shredders, which is likely to be picked up from the soil Hannula et al.
Saprotrophic fungi and bacteria in the soil are involved in the mineralization of nutrients into inorganic forms that can be taken up by plants. How this affects plant species may be strongly determined by the stoichiometric constraints of both plants and the microorganisms involved in soil nutrient cycling Capek et al.
Depending on how plants drive shifts in the composition and activity of detritivores, saprotrophs, and gut microbes, variation in the soil community will contribute to litter-mediated PSFs by altering soil nutrient availability Joly et al.
Also, interactions between decomposers and other soil organisms e. For example, consumption of detritivores and microbial decomposers by predators in the soil could inhibit decomposition Liu et al. Litter-mediated PSFs may also operate via home-field advantage effects, which is the process that litter decomposition is accelerated near the plant where the litter originates from relative to litter decomposition further away from that plant see Gholz et al.
The hypothesis is that home-field advantage effects are driven by species-specific associations between plants and decomposer communities Freschet et al. Recent experimental work showed that different litter types can indeed harbor specific litter microbiomes Keiser et al.
This results in plant-specific patterns in nutrient release Perez et al. In general, home-field advantage effects may be expected to favor the plants where the litter originated from Zhang et al.
Therefore, the extent of plant specificity in litter-induced feedbacks via home-field advantage remains to be tested van der Putten et al. Most studies focus on shoot litter decomposition, but decomposition of root litter may be equally or more important in driving biotic litter-mediated PSFs Li et al. In turn, the mutualistic, pathogenic, and saprotrophic components of the microbial community are consumed by organisms, such as collembola that graze on fungal mycelium Scheu and Schulz, or protists that feed on unicellular bacteria and fungi Radosa et al.
In addition, saprotrophs can compete for root exudates with plant parasites or ectomycorrhiza in the rhizosphere. As a result, both rhizosphere- and litter-mediated PSFs can be controlled by similar trophic top-down and bottom-up factors De Long et al. It is therefore important not to regard them as separate units, but as intertwined operating pathways imbedded in the same food-web context.
Here, we demonstrated how root and shoot litter can affect PSFs through physical, chemical and biotic pathways. Although an emerging body of literature supports the potential importance of litter-mediated PSFs, many critical knowledge gaps remain which require further investigation.
Here we identify five important avenues for further research into the role of litter-mediated PSFs:. We already know that rhizosphere pathogens or symbionts play a strong role in mediating PSFs Bennett et al. Also, microbes in the rhizosphere can immobilize the nutrients liberated from plant litter, thereby buffering litter-mediated PSFs via nutrient cycling Miki et al. These findings indicate the need to study rhizosphere- and litter-mediated PSFs in combination see Box 1 for an example to disentangle their interactions and relative importance.
This will require full-factorial experiments where rhizosphere- and litter-mediated PSFs are manipulated. Within such studies it will be important to carefully consider spatial and temporal scales at which litter- and rhizosphere-mediated PSF effects occur, as these are not necessarily the same. In addition stage-structured , models could be powerful to further explore how litter-mediated PSFs could operate Ke et al. For example, litter cover i. Integrating research across all three pathways will help us to untangle the key mechanisms driving litter-mediated PSFs.
This will require experiments that specifically manipulate the physical, chemical or biotic pathway, for example by using sterilized litter to rule our biotic effects or fake e. For example, plant roots from the first generation of plants can be left in the soil intact to allow for physical, chemical and biotic legacy effects of root litter on next generations of plants.
Evidence is accumulating that plant litters build up unique decomposer communities Lin et al. This may however generate relatively unspecific feedback effects, because different plant species may all profit from nutrients liberated from litter by specialized decomposer communities, however this is not tested. Future work should disentangle to what extent litter-mediated PSFs via the physical, chemical or biotic pathways result in species-specific effects on the growth and performance of next-generation plants.
So far, most work has focused on aboveground litter pathways, but root decomposition may be key in driving litter-mediated PSFs and, importantly, in modifying rhizosphere-mediated PSFs. Please keep in mind that you should not confuse a leaf litter with humus. A leaf litter is just the waste lying on the ground, it turns into humus after it is decayed.
However, it really is important. While there are way too may reasons why it is so, here are some of the chief ones that you might want to know:. Litter aids in soil moisture retention by cooling the ground surface and holding moisture in decaying organic matter. The flora and fauna working to decompose soil litter also aid in soil respiration. A litter layer of decomposing biomass provides a continuous energy source for macro- and micro-organisms. Organic matter quality in ecological studies: theory meets experiment.
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